System and method for suppression of wafer temperature drift in cold-wall cvd systems

ABSTRACT

An apparatus and corresponding method are disclosed that uses one or more optical fibers in a susceptor that monitor radiation emitted by the backside of the susceptor. The optical fibers are filtered and converted into an electrical signal. A control system is used to maintain a constant wafer temperature by keeping the electrical signal constant during the deposition cycle. This overcomes problems caused by varying wafer temperature during non-selective epitaxial and poly-silicon growth on patterned wafers at low temperatures and reduced pressure.

FIELD OF THE INVENTION

The present invention relates to the field of semiconductor wafermanufacturing. More particularly, the invention relates to a method andsystem for controlling and/or compensating for varying wafer temperatureduring non-selective epitaxial growth.

BACKGROUND OF THE INVENTION

Epitaxy is used in the semiconductor industry to grow a layer ofsemiconductor material on top of a single-crystalline semiconductorsubstrate in such a way that the crystal lattice is maintained. Commonexamples are the growth of an epitaxial silicon layer on top of asilicon substrate or the growth of a SiGe alloy layer on top of asilicon substrate. This can be done by chemical vapor deposition (CVD)in a proper environment. The wafer is heated to a suitable temperatureand gases containing the required components are passed over thesubstrate, which usually is a silicon wafer. To avoid growth on thewalls of the reactor vessel it is advantageous to heat just the waferand to keep the reactor walls at a relatively low temperature.

Common epitaxial reactors used in the semiconductor industry consist ofa transparent quartz reactor chamber through which the gas is passed.Inside the quartz chamber the substrate is located on top of asusceptor. The wafer and susceptor are irradiated by high intensitylamps and heated to the desired temperature. As the quartz walls of thereactor chamber absorb comparatively little radiation they can easily becooled. The temperature of the susceptor is measured and controlled; itis assumed that the wafer has approximately the same temperature as thesusceptor.

FIGS. 1 a and 1 b illustrate a simple case. A susceptor 10, usually agraphite disk with a SiC coating, has a pocket 11 machined in its topsurface to hold a wafer (not shown). The temperature of the susceptor 10is measured by a pyrometer 12 aimed at the backside of the susceptor 10.The pyrometer can also be aimed at the top surface of the wafer,directly measuring the wafer temperature. However, since the emissivityof the wafer surface is not constant, especially when patterned wafershave to be deposited, this is not a good method to measure the wafertemperature. In order to improve the uniformity of the deposited layerthe susceptor 10 can rotate during the deposition process.

FIGS. 2 a, 2 b and 2 c show a more complicated case. A susceptor 20consists of two parts: a rotating inner part 21 and a stationary outerpart 22 as shown in FIG. 2 a. The rotating inner part 21 holds a wafer(not shown) in a pocket 23 and rotates. The stationary outer part 22(e.g., a ring) of the susceptor 20 is made of the same material as therotating inner part 21, but does not rotate. This is done to enable atemperature measurement to be done with thermocouple 28 (shown in crosssection in FIG. 2 c).

A small pocket 25 centered at the backside of the susceptor 20 (shown incross section in FIG. 2 b) is intended to receive the tip of thethermocouple 24 as shown in FIG. 2 c. The thermocouple 24 rotatestogether with the rotating inner part 21. The stationary outer part 22holds one or more thermocouples 28 via a thermocouple bore 26 to measurethe temperature at the wafer edge. All the thermocouples 24 and 28 aretied to a temperature control system 27 allowing for a very accuratecontrol of the temperature at the center and the edge of the susceptor20.

While these arrangements have proven useful, significant shortcomingsexist with these arrangements.

SUMMARY OF THE INVENTION

The Applicant of the present invention has realized that there areseveral significant shortcomings of the conventional systems discussedabove. As discussed more below, there is a problem caused by varyingwafer temperature during non-selective epitaxial and poly-silicon growthon patterned wafers at low temperatures and reduced pressure.

One aspect of the present invention is directed to a method andapparatus that uses one or more optical fibers in a susceptor thatmonitor radiation emitted by the backside of the wafer. There areseveral benefits of this type of configuration, e.g.:

-   -   (1) the quality of epi layers improves because dopant and Ge        levels are more stable throughout the layer of a wafer; and    -   (2) tuning efforts to grow a specific layer or layer sequence in        a wafer can be significantly reduced.

One embodiment of the present invention is directed to a wafermanufacturing apparatus including a susceptor including a support for awafer, at least one optical fiber connected to the susceptor so thatradiation from a bottom side of the wafer can be monitored, and anoptical signal measurer coupled to the at least one optical fiber.

Another embodiment of the present invention is directed to a method formanufacturing a wafer using an expitaxy process. The method includes thesteps of receiving an optical radiation signal from a backside of awafer, filtering out a spectrum of the radiation signal for which thewafer is opaque and converting the filtered radiation signal into anelectrical signal. The method also includes the step of controlling awafer temperature by keeping the electrical signal constant during adeposition cycle.

Yet another embodiment of the present invention is directed to a methodto decrease temperature differences between wafers with differentpatterns or different thickness of the field oxide or nitride inepitaxial reactors. The method includes the steps of heating a wafer toa deposition temperature at a first pressure, registering a firstradiation signal level from a backside of the wafer and during asubsequent deposition cycle at a second pressure that is less than thefirst atmospheric pressure, controlling a temperature so that a secondradiation signal level from the backside of the wafer is substantiallyequal to the registered first radiation signal level.

This brief summary has been provided so that the nature of the inventionmay be understood quickly. A more complete understanding of theinvention can be obtained by reference to the following detaileddescription of the preferred embodiments thereof in connection with theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a prior art susceptor in an epitaxial reactor.

FIG. 2 depicts another prior art susceptor in an epitaxial reactor.

FIG. 3 depicts a susceptor in accordance with a preferred embodiment ofthe present invention.

DETAILED DESCRIPTION

The assumption that a wafer is at approximately the same temperature asa susceptor is sufficiently close to reality at high temperatures. Inthis regard, high temperatures being defined as a temperature regime inwhich the growth rate is “transport limited.” The diffusion of freshreactants to the surface of the wafer is the limiting factor and in thisregime the growth rate is only moderately dependent on the temperatureat high temperatures. Above 850-900° C. in case of silicon epitaxy, thesystems described above may provide satisfactory results. However, atlow temperatures this is no longer the case. There are two reasons whythe susceptor temperature is an insufficient measure of the wafertemperature in the low-temperature range. In this regard,low-temperatures being defined as a temperature regime in which thegrowth rate is not “transport limited.”

First, it is noted that at low-temperatures, the growth rate of thewafer now is a strong function of the temperature. At low temperatures,for silicon epi below 850° C., the growth rate is determined by thereaction rate. The growth is said to be in the “kinetically controlled”regime, characterized by a strong temperature dependency. In this case,a temperature difference between the wafer and susceptor is much morenoticeable at low temperature than it is at high temperature.

Next it is noted that at lower temperatures, the heat transfer betweenthe wafer and susceptor shifts from being dominated by radiation tobeing dominated by conduction. A thin gas film formed between thesusceptor and the wafer can play an important role and the heat transferdecreases when the pressure is reduced. This implies that there can be asubstantial temperature difference between the wafer and the susceptorat low temperatures and at reduced pressure.

As discussed in the “Emissivity Effects in Low-Temperature EpitaxialGrowth of Si and SiGe”, W. B. de Boer et al., Electrochemical SocietyProceedings Vol. 99-10, incorporated herein by reference, thetemperature difference (between the wafer and the susceptor) is not themajor problem. The problem is that during the growth on patternedwafers, the emissivity of the wafer changes. This changes the heat fluxbetween wafer and susceptor, which in turn causes the wafer temperatureto change during the growth. This rather unpredictable temperature driftmay cause one or more harmful effects:

-   -   i) the growth rate changes during the deposition, the layer        thickness of the wafer does not increase linearly with the        deposition time;    -   ii) the incorporation of, e.g., B and P is not constant and        changes during the deposition;    -   iii) when growing SiGe alloy layers, the Ge content varies        during the deposition; and    -   iv) when switching to a different pattern or a different        thickness of field oxide or nitride, the wafer temperature can        be different and one has to go through an elaborate retuning        procedure.

The varying emissivity of the wafer and the more constant emissivity ofthe susceptor edge around the wafer also causes a center-to-edge effecton the wafer. The temperature at the center changes differently from thetemperature at the edge of the wafer during the growing cycle, i.e., thecenter-to-edge variation of the layer thickness and dopant concentrationchanges with increasing layer thickness and changes with the pattern.

The problems described here are characteristic of so-called cold-wallreactors and are irrespective of the heating method. In this regard, dueto the high temperature dependency of the growth rate and of parameterslike B and Ge incorporation into the growing layers, the problemsmanifest themselves, for example, when blanket layers are grown onpatterned wafers at low temperature and reduced pressure. During theblanket deposition of silicon, an epitaxial layer is deposited inwindows where underlying silicon is exposed and a poly-silicon (orpoly-SiGe) layer is deposited on top of the oxide or nitride. It is thegrowth on top of the oxide and/or nitride layers usually covering morethan 90% of the wafer surface, which changes the emissivity of thewafer, resulting in temperature variations.

One possible method to address the problem described above would be tomeasure the true wafer temperature, e.g. with a pyrometer aimed throughthe quartz reactor wall at the wafer with proper optics. However, thishas not been possible in the present generation of epitaxial reactorsfor various practical reasons. The fundamental problem is that theemissivity of the top surface of the wafer is changing during thegrowth, especially when patterned wafers are deposited of course. Theradiation energy picked up by the pyrometer can only be translated intoa temperature when the emissivity of the radiating subject is known.Consequently, the pyrometer reading of the radiating top surface of awafer which emissivity is changing cannot be converted into atemperature unless the emissivity is known, which is not the case inthis arrangement.

Other difficulties, less fundamental but still problematic, are that thepyrometer (1) tends to pick up scattered radiation from heating lampsand (2) quartz process tubes tend to get a thin coating during thedeposition. Both of these effects tend to disturb an accuratetemperature reading. As should be appreciated, measuring the susceptortemperature in contrast to measuring the wafer temperature, by means ofa pyrometer is possible but less than ideal and sensitive to drift.

One embodiment of the present invention is directed to circumventingthese difficulties and at the same time solving the problem of thevarying wafer temperature. This is accomplished by inserting an opticalfiber in a susceptor underneath a wafer in such a way that it receives(monitors) radiation from the backside of the wafer.

It is noted that an optical fiber inserted in a susceptor may be used ine.g. sputtering and Reactive Ion Etch (RIE) equipment. The purpose ofthe optical fibers in this case, however, is wafer temperaturemeasurement for which the backside emissivity has to be known. Inaddition, optical temperature measurements, using pyrometers and/oroptical fibers are common in Rapid Thermal Processing (RTP) equipment.But again in this case, this arrangement only considers true wafertemperature measurements for which the emissivity of the wafer has to beknown.

The optical fiber is to be coupled to a measuring device. Such measuringdevices are well known in the art and are not described in detailherein. A section from the spectrum for which the wafer is opaque isfiltered (band-pass filter). The optical signal is then converted intoan electrical signal using a transducer.

To maintain a constant wafer temperature, the electrical signal shouldbe kept constant during the deposition cycle. This is easily achieved bya dedicated control system. The backside of the wafer may or may not becoated. The only requirement is that the optical signal is kept constantfrom the start of deposition.

It is noted that the conventional susceptor temperature measurementsystem (i.e., element 12 in FIG. 1 and elements 24, 28 and 27 in FIG. 2)described above is not modified. It is also noted that the emissivity ofthe backside of the wafer does not play a role in this arrangement.

In another embodiment, one or more optical fibers may be inserted intothe susceptor. In this arrangement, one optical fiber monitors a centerof the wafer and a second optical fiber monitors an edge of the wafer.By keeping both optical signals constant from the onset of thedeposition, the center to edge variations can also be suppressed. It isnoted that the emissivity of the wafer backside at the center and at theedge of the wafer does not need to be the same. Inserting additionaloptical fibers is also possible, but the improvement in performanceshould be balanced with the increased complexity of the measure/controlsystem.

Those skilled in the art will also realize that the solution to thewafer temperature varying during the deposition cycle, as describedhere, can also be used to decrease temperature differences that exist inthe conventional (epitaxial) reactors between wafers with differentpatterns or different thickness of the field oxide or nitride.

Since the conventional temperature measurement system (described above)has not been modified, it can be used in conjunction with variousembodiments of the present invention to calibrate the wafer temperature.The procedure is as follows: a wafer is heated to a depositiontemperature at a first pressure (e.g., an atmospheric pressure). In thisconfiguration, the thermal coupling between the wafer and the susceptoris good and the temperature difference between wafer and susceptor issmall. The signal levels from the optical sensors are registered andduring the subsequent deposition cycle at reduced pressure thetemperature has to be controlled in such a way that the signals of theoptical sensors are substantially restored to the registered value. Thisprocedure calibrates the wafer temperature regardless of the pattern andwill result in substantial savings in setting up the reactor for waferswith different masks and different thickness of dielectric.

FIGS. 3 a, 3 b and 3 c are block diagrams of a susceptor 30 inaccordance with a preferred embodiment of the invention. The susceptor30 consists of two parts: a rotating inner part 31 and a stationaryouter part 32 as shown in FIG. 3 a. The rotating inner part 31 holds awafer 33 in a pocket 34. It is noted that this block diagram shows acomplicated case, i.e., an arrangement that uses thermocouples 24 and 28for the temperature measurement. Simpler arrangements may also be usedfor the susceptor 30, e.g., without the thermocouples 24 and/or 28 orwith only one optical fiber.

As shown in FIG. 3 b, two holes 35 and 36 are drilled through thesusceptor 30. One hole 36 is located near the center of the susceptor 30and the other hole 35 is located near the edge of the wafer 33. Opticalfibers 37 and 38 are inserted in the respective holes 35 and 36 so as topick up the radiation emitted by the backside of the wafer 33 (FIG. 3c). The optical fibers 37 and 38 can, for example, be made of sapphire(Al₂O₃) when very high temperatures are used during the process orquartz (SiO₂) when the temperatures are in the moderate range. Theoptical fibers 37 and 38 may also be integrated in the (quartz)structure that supports the susceptor 30 (not drawn).

The optical fibers 37 and 38 are coupled to a measuring device 39. Asection from the spectrum for which the wafer is opaque (band-passfilter) is filtered. The optical signals are then converted into anelectrical signal using a transducer 40, which provides a feedbackcontrol signal. A control system 43 receives the feedback control signaland maintains, for example, a constant wafer temperature during adeposition cycle in accordance with the feedback control signal.

The susceptor 30 rotation complicates the coupling of the optical signalto the measuring device 39. Rather than transforming the optical signalsfrom the optical fibers 37 and 38 into electrical signals (e.g., beforetransmission over sliding contacts), as is done with the thermocouple 24signal, it is preferable to optically couple the rotating optical fibers37 and 38 to a stationary monitoring device 41. During one susceptor 30revolution, each of the optical fibers 37 and 38 transmit opticalsignals during a short period of the rotation cycle to the monitoringdevice 41 when passing by, via an optical multiplexer 42. This allowsfor the number of components/parts of the overall system to be minimizedand also allows both signals from the optical fibers 37 and 38 to followthe same optical and electrical path, which reduces the influence ofdrift in the signal-processing path.

The present invention has been described with respect to particularillustrative embodiments. It is to be understood that the invention isnot limited to the above-described embodiments and modificationsthereto, and that various changes and modifications may be made by thoseof ordinary skill in the art without departing from the spirit and scopeof the appended claims.

1. A wafer manufacturing apparatus comprising: a susceptor including asupport for a wafer, the wafer including a topside and a bottom side; atleast one optical fiber connected to the susceptor so that radiationfrom the bottom side of the wafer can be monitored; and an opticalsignal measurer coupled to the at least one optical fiber.
 2. The wafermanufacturing apparatus according to claim 1, wherein two optical fibersare connected the susceptor, a first optical fiber being located near acenter of the susceptor and a second optical fiber being located near anedge of the wafer.
 3. The wafer manufacturing apparatus according toclaim 1, wherein the optical signal measurer filters an optical signalfrom the at least one optical fiber, converts the filter optical signalinto an electrical signal and provides a feedback control signal.
 4. Thewafer manufacturing apparatus according to claim 1, wherein the at leastone optical fiber is inserted into a hole in the susceptor to access thebottom side of the wafer.
 5. The wafer manufacturing apparatus accordingto claim 1, wherein the at least one optical fiber comprises sapphire.6. The wafer manufacturing apparatus according to claim 1, wherein theat least one optical fiber comprises quartz.
 7. The wafer manufacturingapparatus according to claim 1, wherein the optical fiber is integratedin a structure that supports the susceptor.
 8. The wafer manufacturingapparatus according to claim 1, wherein the susceptor includes arotating part and a stationary part.
 9. The wafer manufacturingapparatus according to claim 8, further comprising a thermocouple or apyrometer arranged to measure a temperature of the susceptor.
 10. Thewafer manufacturing apparatus according to claim 8, wherein opticalsignals from the at least one optical fiber couple to the optical signalmeasurer via a stationary monitoring device.
 11. The wafer manufacturingapparatus according to claim 1, further comprising a control system thatreceives the feedback control signal and maintains a constant wafertemperature during a deposition cycle.
 12. A method for manufacturing awafer using an expitaxy process, the method comprising the steps of:receiving an optical radiation signal from a backside of a wafer;filtering out a spectrum of the radiation signal for which the wafer isopaque; converting the filtered radiation signal into an electricalsignal; and controlling a wafer temperature by keeping the electricalsignal constant during a deposition cycle.
 13. The method according toclaim 12, wherein the receiving step includes receiving first opticalradiation signal from a center of the wafer and a second opticalradiation signal from an edge of the wafer.
 14. The method according toclaim 13, wherein the controlling step includes keeping the first andsecond optical radiation signals constant from an onset of thedeposition.
 15. A method to decrease temperature differences betweenwafers with different patterns or different thickness of the field oxideor nitride in epitaxial reactors, the method comprising the steps of:heating a wafer to a deposition temperature at a first pressure;registering a first radiation signal level from a backside of the wafer;during a subsequent deposition cycle at a second pressure that is lessthan the first pressure, controlling a temperature so that a secondradiation signal level from the backside of the wafer is substantiallyequal to the registered first radiation signal level.
 16. The methodaccording to claim 15, wherein the subsequent deposition cycle creates adifferent pattern or different thickness of a field oxide or nitride onthe wafer.
 17. The method according to claim 15, wherein the firstpressure is an atmospheric pressure.